Teacher - I am the addict of all Diet Coke addicts. I can down a good 10 or 12 a day. How's that for addiction !!! Anyhow, You can get actual Diet Coke in cans at Island Supermarket south of town by Ramon's or at San Pedro Super Market up North by the Blue Tang Inn.
Coke lite is pretty comparable to Diet Coke and satifies my addiction. I suggest if maybe you can contact your hotel or place of lodging to see if the local distributor can deliver a crate/case to your room and save you the hassle. Most places will do this with sodas/beer as the distributor on the island has a monopoly on coke products and beer and will gladly supply you. It also cheaper than the grocery stores this way.
Last I paid about 75 cents for a can of the real stuff and less for Coke Lite.
Have one or two sodas in the morning to get you going, then switch to Belikin or venture to BC's Bar and have Ernie pour you the Drink D'Jour and you won't miss Diet Coke at all nor care !